Page 12 - Ragg_et_al-1999-Tectonics

680 RAGG ET AL.: TECTONIC STRESSIN SICILY

[A] TOP VIEW sedimentg, raniteand gabbrolayers,while the oceaniccrust
hasgabbroandsedimenltayers.In theforearcbasinareasthe
SOUTH NORTH sedimentarycover is split into a carbonateand a slightly
A' A compactedsedimenltayer (Fig 1lb). The differentrocktypes
are shown by different shadingsin Figure 11, and the
B' B correspondingmaterial properties are listed in Table 3. To
take into accountthe Malta Escarpment,the crustwas modeled
Pantelleria Riff Caltanissetta Basin as a very weak zone with a stronglyreducedYoung's modulus,
Calabrian Arc which was 1% the value of the surroundingmaterial [Spann et

SedimentsCARBONTAES Metamorphic al., 1994].
The translative degrees of freedom of the nodes at the
Gabbro
bottom (the Moho) are fixed in vertical direction. The
[B] 480 km horizontal boundary conditions are set up by displacements
MODEL CROSS SECTIONS (see arrows in Figure 1l a). The displacementsat the model
boundariesare chosenin sucha way that a uniform NNW-SSE
Figure 11. (a) Top view of the model.The differentshading direction of SHmax is obtained for a homogeneousreference
indicatesdifferent rock types as given on section C-C'. The model with no variation in thicknessand material properties.
The far-field stress orientation along the northern boundary
white lines indicate the cross sections. The arrows show the will be readjustedby regional crustalstressescausedby mantle
diapir in the Tyrrhenian back arc basin.For uniform boundary
displacementsapplied at the model boundaries.(b) N-S cross conditions this will be only modeled by the Moho
sectionsof the model for westernSicily (A-A'), centralSicily topographywithout additional displacementdue to back arc
(B-B'), and the Ionian Sea (C-C'). The depth is four times
exaggerated. extension.

This NNW-SSE far-field stress orientation is in accordance

with the observed stressdata [World Stress Map Project;
Zoback, 1992] and is interpreted to result from the collision
of the Europeanand African plates [Richardsonet al., 1979;
Miiller et al., 1992]. The displacementswere computedfrom
geodeticstrain rate measurements[Ward, 1994] for the last 1
million years. Since we use a linear elastic model, the stress
orientationis dependenton the ratio of the displacementsat
the model boundariesbut independentof the absolutevalues.
Also, the opening of the Pantelleria Rift was modeled by
displacements perpendicular to the rift axis. The
displacements of 1 km represent the opening of the
Pantelleria Rift in the last 1 million years [Reuther and
Eisbacher, 1985; Reuther et al., 1993].

The model allows us to discriminate between the influence

of materialpropertiesand geometryand contributionfrom rift
opening(1 km). For example, the model Siz36 was calculated
with all elementssetto the samematerialproperties(Young's
modulus (80 GPa) and Poisson's ratio (0.25)) and without rift

opening.Thus the model consistsof a varyingMoho depthin
a homogeneousfar-field stress,and the patternof the SHmax
directionsis due to the variation of Moho depth. In this model
SHmaxrotatesup to 40ø clockwisefrom the far-field direction
in the area of northeasternSicily and Calabria, and it rotates
-30øcounterclockwisein central and western Sicily (Figure
12).

In additionto the variation of the Moho depth, model siz50
(Table 4 and Figure 12) takes into account the material
propertiesof the different crust types. However, the rather
moderatechangesin the Young% modulusdo not causemajor
modificationsin comparisonto Siz36.

In the next step (model siz35, Table 4 and Figure 12 ) the
openingof the Pantelleria Rift System was consideredby a
NE-SW displacementof the elements off the rift axis. This